The superheterodyne receiver is a commonly used receiver to step-down and recover (i.e. demodulate) a baseband signal from a received radio frequency (RF) signal. A first stage mixer is used to step down the RF signal to an intermediate frequency (IF) signal, and a demodulator is used to recover the baseband signal from the IF signal. The demodulator can include a second stage mixer that is used to step down the IF signal and recover the baseband signal. For in-phase (I) quadrature (Q) demodulation the second stage can use two mixers.
A common problem with superheterodyne receivers is that mixers are not ideal and can provide unwanted spurious outputs (i.e. spurs) and images that may require complex circuitry to filter. In some applications images and spurs may fall within the frequency range of the baseband signal. One technique to reduce the spectral energy of the spurs is to increase the local oscillator (LO) power of the offending mixer. Another technique is to reduce the RF input power. However, both techniques may degrade the overall signal-to-noise ratio (SNR) of the receiver. Narrowband filtering of the RF input and the down-converted signal may also be used to attenuate spurs and unwanted images at a cost of bandwidth of the receiver. Image rejection mixers can be used as well. However, temperature stability can be an issue with image rejection mixers. More limiting and complex techniques involve digitally capturing the output spectrum of the mixer at two offset LO frequencies. By observing frequency shifts, certain spurs can be identified and removed mathematically by software. A major disadvantage to this technique is that it requires batch mode processing and is not practical for real-time applications.
Spurs can also be generated when using time interleaved analog-to-digital converters (ADCs) in receivers. These types of spurs can be attenuated by measuring time and frequency errors and then compensating for these errors with a programmable finite impulse response (FIR) filter. However, these errors tend to be unstable and also sensitive to temperature. A calibration signal is required and adds complexity to the circuitry. Also, digital programmable FIR filters require high gate counts in the application specific integrated circuit (ASIC), field programmable gate array (FPGA), or digital signal processing (DSP) platforms.
For at least the aforementioned reasons, there is a need for improved techniques for reducing spectral energy of spurs and unwanted images generated in RF receivers that fall within the frequency range of the baseband signal.